Miniature cartridge fuses commonly include a main insulating housing made of a rigid material which is sometimes glass, but more commonly a stronger material, like a ceramic material. In fast blowing fuses, a fuse wire generally extends diagonally across a longitudinal housing passageway, with the wire ends bent back over the housing ends. In slow blowing fuses, the fuse wire may be a winding of fuse wire on an insulating core extending axially of the housing passageway. The ends of this fuse wire winding may be bent back over the housing ends. Cup-shaped end caps extend over the ends of the housing and the bent back ends of the fuse wire fit between the side walls or flanges of the end caps and the housing exterior. Typically, the ends of the fuse element are electrically and physically secured to the end caps by a body of solder in each of the end caps. The solder generally extends into small clearance spaces between the flanges of the end caps and the housing.
To prevent the end caps from falling off the housing under normal handling conditions and under short circuit blowout conditions, a shrink sleeve or an encapsulation material has been applied around the housing and the end caps. The encapsulation material is preferably applied in a manner which also forms a seal around the end caps and the inner end portions of any outwardly extending leads, if provided, to seal the interior of the fuse from printed circuit board cleaning chemicals where the fuse is used on printed circuit boards. The elimination of solder and the shrink sleeve or encapsulation would significantly reduce production costs. The fuses of the present invention, which do so, are useful where cost considerations are of paramount importance for a fuse of a given desired interruption or blowout capacity. Also, it is desired that low cost by well known or other mass on techniques.
Efforts have been made to produce solderless cartridge fuses. As disclosed in U.S. Pat. No. 3,962,668, one such prior art solderless fuse uses a compressible housing made of a cellulose material where the side walls of flanges of cup-shaped end caps are forced over the ends of the housing so that the housing deforms under the force applied by the end cap flanges. The fuse wire has bent back ends held between the end cap flanges and the housing ends. The use of such a housing material is undesirable for a number of reasons. One reason is that such a housing does not have as much strength to withstand high overload currents without exploding and thus has modest overload current capabilities.
Another form of a solderless cartridge fuse is shown in U.S. Pat. No. 2,876,312. It uses a rigid, unyielding housing and cup-shaped end caps with flanges crimped over the outer ends of the housing to compress the bent back ends of the fuse wire between the end cap flanges and the housing. This type of cartridge fuse construction is undesirable because, among other reasons, the crimping of the end caps around the housing ends can cause cracking of the housing, especially in miniature fuses where the housing walls are typically very thin.
One type of fuse which is commonly solderless because of its nature is a renewable cartridge fuse. Such a fuse has removable end caps so that a blown fuse wire can be replaced so the fuse can be re-used. Because of cost and size requirements of such a fuse, and the inherent necessity that the end caps and fuse wire be readily removable and hence free of solder connections, the design details of such a fuse for the most part are not applicable to the one fuse cartridge fuses of the present invention. An example of such a solderless renewable fuse is disclosed in U.S. Pat. No. 1,485,211 to Berger, granted Feb. 26, 1924. It discloses a solderless fuse which includes cylindrical end caps with tightly press against bent back ends of a fuse wire pressed by the skirt portion of the end caps into tapered recesses in the end portions of the fuse housing. The end wall of each end cap bears against the portion of the end of a fuse wire centered in the fuse housing which bends at right angles to the fuse axis as it leaves the housing passageway.
As will appear, the present invention utilizes a technique of assembly which heretofore was believed to be an assembly method which would in most cases break the fuse wire. Especially in miniature fuses which use very fine filament wire, an interference fit between the end cap flanges and the housing in the presence or absence of the fuse wire was not considered feasible, since the assembly of the end caps over the housing would be expected to stretch and break the fuse wire. This problem was of concern to the patentee of U.S. Pat. No. 4,920,327.
To avoid this problem, in the fuse disclosed in this patent recesses were placed in the housing periphery at the ends thereof into which the bent back ends of the fuse wire extended, so that the base portions of these bent back portions of the fuse wire would not be engaged and thereby stretched as the end caps of the housing are pushed over the housing ends. This fuse, however, utilized solder in the end caps. For this reason and the presence of the recesses the cost of the fuse is significantly greater for a given fuse size and interruption capacity than fuses made in accordance with the various forms of the present invention.